1. Field of the Invention
The present invention relates to a scanning electron microscope with a length measurement function, and a sample dimension measurement method to observe and measure a sample by irradiating an electron beam onto the sample.
2. Description of the Prior Art
In the processes of manufacturing semiconductor devices, observation of a sample, measurement of a line width of a pattern and the like are performed by using an electron beam device such as an electron microscope. In observation and measurement of a sample performed by the electron beam device, a scanning operation is performed in the following manner: while an electron beam is irradiated onto a portion of the sample, an amount of electrons such as secondary electrons are converted into luminance so as to display an image on a display device.
As described above, when an observation and measurement of a sample is performed, an electron beam is irradiated thereon. The irradiation of electron beam, however, creates a phenomenon in which a surface of the sample is electrified (electrically charged or charge-up). In other words, depending on a difference between the amount of charged particles incident on a sample and those discharged therefrom, an irradiated surface is electrified positively or negatively. Depending on an electrification potential on a surface of the sample, discharged secondary electrons are accelerated or brought back to the sample, so that an efficiency of the discharging of the secondary electrons changes. Consequently, a problem occurs in which a detected amount of electrons becomes unstable.
To deal with such a problem, in Japanese Patent Application Laid-open Publication No. Hei 10-213427, a method is disclosed in which damage on a circuit pattern and a charge-up thereof is reduced and a dimension of the circuit pattern is measured.
As described above, when a sample is observed by using an electron beam device, a phenomenon in which the sample is electrified occurs. On the other hand, for example, for a sample such as a wafer which is electrically connectable, an electric conductor on an electrically connected wafer is grounded and thereby can be discharged, so that the electrification level of the sample can be reduced. Thus, the electrification does not practically pose any problem.
However, for example, in a state where a process of wiring in manufacturing has been completed, there sometimes exists an isolated portion of a circuit pattern having electrically conductive wiring on a glass substrate, or exists a sparse or dense portion in an electrically conductive wiring.
When such a pattern formed on a semiconductor wafer or on a photomask is measured by using an electron beam device, an electric potential distribution of a surface of a measurement target varies depending on the shape of a pattern, a wiring width, a material, a film thickness, etc. on the measurement target. For example, even when each of the patterns has the same wiring widths, surface potential distributions vary depending on whether intervals at which patterns are formed are small or large. More specifically, the larger the intervals at which the patterns are formed, the higher it becomes the surface potential of the pattern. When the line width of the pattern of a measurement target is increased, a phenomenon occurs in which dimension error becomes large.
As described above, depending on the width of the wiring and the like, a charge-up occurs where a charged amount becomes locally different from one another. Because of the influence of this charge-up, a primary beam is deflected so that a length measurement magnification is changed, thus causing a dimension measurement error.